High range resolution light detection and ranging

ABSTRACT

Lidar systems and methods are presented herein. In one embodiment, a lidar system includes a laser operable to propagate ultrashort laser pulses to a target during a plurality of scanning periods. The lidar system also includes a streak tube imaging system operable to collect returns of the ultrashort laser pulses from the target during each scanning period, and to generate a two-dimensional image of the returns during each scanning period. The lidar system also includes a processor operable to generate a representation of the target based on the 2D images from the streak tube imaging system.

CROSS REFERENCE TO RELATED APPLICATIONS

This patent application claims priority to, and thus the benefit of anearlier filing date from, U.S. Provisional Patent Application No.62/589,251 (filed Nov. 21, 2017), the entire contents of which arehereby incorporated by reference.

BACKGROUND

Light Detection and Ranging, or “lidar” (also referred to as LaserDetection and Ranging, or “LADAR”) typically involves propagating pulsesof laser light to an object and measuring the time it takes for thepulses to scatter and return from the object. For example, since lightmoves at a constant and known speed (˜3×10⁸ meters per second in air), alidar system can calculate the distance between itself and the objectbased on the amount of time it takes for each pulse to bounce back fromthe object.

SUMMARY

Lidar systems and methods are presented herein. In one embodiment, alidar system includes a laser operable to propagate ultrashort laserpulses to a target during a plurality of scanning periods. An ultrashortlaser pulse is a laser pulse that is generally on the order of afemtosecond to ten picoseconds in duration. The lidar system alsoincludes a streak tube imaging system operable to collect returns of theultrashort laser pulses from the target during each scanning period, andto generate a two-dimensional (2D) image of the returns during eachscanning period. A processor then generates a representation of thetarget based on the 2D images from the streak tube imaging system.

The various embodiments disclosed herein may be implemented in a varietyof ways as a matter of design choice. For example, some embodimentsherein are implemented in hardware whereas other embodiments may includeprocesses that are operable to implement and/or operate the hardware.Other exemplary embodiments, including software, firmware, hardware, andvarious combinations thereof are described below.

BRIEF DESCRIPTION OF THE FIGURES

Some embodiments are now described, by way of example only, and withreference to the accompanying drawings. The same reference numberrepresents the same element or the same type of element on all drawings.

FIG. 1 is a block diagram of an exemplary lidar system.

FIG. 2 illustrates an exemplary curved detection sweep path of a laserpulse return on a 2D image plane.

FIG. 3 illustrates the range image extracted from the exemplary curveddetection sweep path of FIG. 2.

FIGS. 4 and 5 illustrate an exemplary lidar illumination of a sphericalobject.

FIG. 6 illustrates an exemplary mode-locked fiber laser with a ringcavity.

FIG. 7 shows an exemplary crossing deconfliction.

FIG. 8 shows an exemplary X-ray computed tomography geometry.

FIG. 9 illustrates an exemplary direction of the propagating radiationand an accumulated Radon transform for a specific angle associated withthe direction of the signal and a computed target image calculatedthrough an inverse Radon transform.

FIG. 10 demonstrates the effect of measurements over a limited angularextent.

FIG. 11 is a block diagram of another exemplary lidar system.

FIG. 12 is a flowchart of an exemplary process of the lidar system.

FIG. 13 is an exemplary block diagram of a mode-locked laser cavity.

FIG. 14 is an exemplary series of pulses from a mode-locked lasercavity.

FIG. 15 exemplarily illustrates laser pulses and their returns.

FIG. 16 exemplarily illustrates laser pulse returns and theircorresponding effects on a streak tube imaging system.

FIG. 17 is a block diagram of another exemplary lidar system employing asupplemental detection system.

FIG. 18 is a block diagram of another exemplary lidar system in oneexemplary reduction to practice.

FIG. 19 shows a series of targets identified by the exemplary lidarsystem of FIG. 18 in one exemplary reduction to practice.

FIG. 20 shows range images that were extracted from a photon countingprocessed image frame that were stacked to form a sparse target Radontransform in one exemplary reduction to practice, and a computed targetimage produced through an inverse Radon transform.

FIG. 21 is a block diagram of an exemplary computing system in which acomputer readable medium provides instructions for performing methodsherein.

DETAILED DESCRIPTION OF THE FIGURES

The figures and the following description illustrate specific exemplaryembodiments. It will thus be appreciated that those skilled in the artwill be able to devise various arrangements that, although notexplicitly described or shown herein, embody the principles of theembodiments and are included within the scope thereof. Furthermore, anyexamples described herein are intended to aid in understanding theprinciples of the embodiments and are to be construed as being withoutlimitation to such specifically recited examples and conditions.

FIG. 1 is a block diagram of an exemplary lidar system 10. The lidarsystem 10 employs a laser 8 that may be a mode-locked laser to producerelatively short optical pulses (e.g., laser pulses that are less than 1ns in duration) at a high repetition rate (e.g., a pulse repetitionfrequency, or PRF, of at least 1 MHz). The laser 8 delivers a stream oflaser pulses 6 to a target 7, which may be at a distance that is muchlonger than the separation distance between the delivered optical pulses6.

The lidar system 10 also includes a streak tube 11 to provide relativelyhigh range resolution. Streak tubes may be used to map space-timedistributions of received optical signals (e.g., photons). The opticalsignal may be received through an aperture 17 and mapped to a 2D image.Typically, streak tubes are operated so that a horizontal axis of theimage corresponds to a spatial position of the signal along a slitaperture, and a vertical axis of the image corresponds to the time delayof the signal. The image intensity corresponds to input opticalintensity of the signal. However, in this embodiment, the streak tube 11may be configured with an aperture 17 that is somewhat symmetric in bothvertical and horizontal directions (e.g., circular). This may providethe lidar system 10 with improved resolution of the target 7 at greaterdistances.

In this embodiment, the streak tube 11 receives optical signals 16(e.g., laser pulse returns) with a photo-cathode 14. The aperture 17couples the optical signals 16 to the photo-cathode 14. For example, theaperture 17 may image laser pulse returns 16 from the target 7 onto thephoto-cathode 14. And, light patterns appearing at the aperture 17illuminate the photo-cathode 14 and cause the photo-cathode 14 torelease electrons 18 in a pattern that matches the light pattern.

Generally, in the streak tube 11, a scan in two dimensions is preferableso as to distinguish individual returns 16 from one another. Thus, thelidar system 10, in this embodiment, comprises two electrode pairs 13-1and 13-2 that may circularly displace (or displace in some othercontinuous pattern, such as a Lissajous pattern) the electrons 18 asthey pass through the streak tube 11 depending on the voltage waveforms9-1 and 9-2. Although, in some embodiments, a single electrode pair 13may be used to displace the electrons 18.

The voltage waveforms 9-1 and 9-2 placed on electrode pairs 13-1 and13-2 provide transverse displacements to the electrons 18 and direct theelectrons 18 towards transverse coordinates on the phosphor screen 15,where the electron's kinetic energy is converted into photons 19 in aspatial pattern matching the light temporal patterns at the aperture 17.A sweep voltage circuit 12 produces a dynamic electric field across theelectrode pairs 13-1 and 13-2 using time varying voltages 9-1 and 9-2.In some embodiments, magnetic field-based electron deflection may beused.

Generally, the voltage difference across the electrode pairs 13-1 and13-2 may result in an electric field which places forces on electrons 18propagating nearly perpendicular to the field. The two electrode pairs13-1 and 13-2 may be used to control displacement of the propagatingelectrons 18 in both the vertical and horizontal directions of thephosphor screen 15. For example, each electrode pair 13-1 and 13-2 maybe coupled to a dedicated time varying voltage waveform source (e.g.,voltage waveforms 9-1 and 9-2) to sweep electrons 18 resulting from thereturns of the ultrashort laser pulses 16 in a continuous pattern. Thevoltage waveforms 9-1 and 9-2 provided to the streak tube 11 by thesweep voltage circuit 12 via the electrode pairs 13-1 and 13-2 provide atime-dependent location on the phosphor screen 15 (or other electronreceiving component) where the electrons 18 are incident. In thisregard, the voltage waveforms 9-1 and 9-2 may scan the electron beamacross the phosphor screen 15 in a manner illustrated in FIG. 2.

This electric field deflects the path of the electrons 18 traversing thestreak tube 11 such that a phosphor image is formed at the phosphorscreen 15 where electrons 18 impact the screen 15. For example, a laserpulse return 16 received by the streak tube 11 at one moment will bedisplaced in the phosphor image with respect to an earlier momentforming an illuminated shape in the phosphor image on the arc 33 (e.g.,illuminations 34 with only a few shown for the sake of brevity). Thetime delay of the laser pulse returns 16 illuminating the photo-cathode14 is generally proportional to the distance within the backscatteringmedium (e.g., the target 7) and results in a displacement offset in thephosphor image formed by the phosphor screen 15.

A resultant 2D spatial emission pattern at the phosphor screen 15 isdigitized using an imaging optical element 20 and a detector 21, such asa lens configured with a focal plane array (FPA), an optical detectorarray lens, or other imaging device. The image, therefore, has atemporal-sensing direction or path corresponding to a time delay and mayhave an additional spatial-sensing path from the aperture 17corresponding to an intensity on the phosphor screen 15. The imagingoptical element 20, if configured as a lens, provides a means formapping the phosphor image to the imaging detector 21. However, othertechniques may be used, such as tapered fiber arrays, direct electronreceiving detector arrays for electrical signal readout, imaging lenses,and/or focusing elements

To illustrate, the lidar system 10 may emit a beam of laser pulses 6 toilluminate the target 7. The streak tube 11 may be aligned so that thelaser pulse returns 16 may be imaged onto the aperture 17. The lightpattern of the returns 16 from the phosphor screen 15 is imaged onto anFPA (e.g., the detector 21) for analysis. The aperture 17 maysubstantially limit the laser pulse returns 16 to a point on thephoto-cathode 14. The processor 26 can determine and form a structuralrepresentation of the target 7 (e.g., a 2D image, a three-dimensional“3D” image, or some other identifying signature of the target 7) fromthe intensities of the returns 16.

The detector 21 may also generate synchronization triggers that areproduced with dynamically controlled temporal displacements relative tothe optical pulses 6 from the laser 8. In this regard, the processor 26may synchronize the propagation of at least a portion of the ultrashortlaser pulses 6 from the laser 8 with a corresponding portion of thereturns of the ultrashort laser pulses 16 at the streak tube 11. Thesweep voltage circuit 12 may also produce voltage waveforms for streaktube deflection that are synchronized to laser triggers provided by thelaser 8. In one embodiment, the two waveforms 9-1 and 9-1 are generatedwith sinusoidal shapes and with a 90-degree phase delay between the two.This results in a circular scan pattern of the propagated electrons 18at the phosphor screen 15.

However, other waveforms 9-1 and 9-1 may be used to scan the electrons18 in other patterns, such as Lissajous patterns. For example, onewaveform 9 may be periodic with a frequency that is an integer multipleof the laser pulse frequency, so that, in general, the electron sweeppattern may be a closed Lissajous pattern that repeats at the laserrepetition frequency (or a harmonic thereof). The relative phase delaysbetween the laser triggers and the phases of each waveform are a matterof design choice and may be selected to distribute the time-dispersedsignal in a preferred pattern upon the phosphor screen 15.

To illustrate, FIG. 2 shows the plane 30 of the phosphor screen 15 wherethe streak tube 11 scans a circular scan pattern, as illustrated by arc33. Electrons hitting the phosphor screen 15 and causing illuminationare associated with received lidar photons having a delay time that isproportional to the angle θ (31) of the circular scan pattern. Byanalyzing signal strengths of image pixels that are in the path of thecircular scan pattern and associating them with the angle θ, a rangeimage can be extracted.

FIG. 3 illustrates the range image 40 extracted from the exemplarycurved detection sweep path of FIG. 2. The range image time is τ=θ/Γ,where Γ is the frequency of the streak tube circular scan. Because thescan is circular, the captured range image 40 is cyclic with a timeperiod equal to 2π/Γ (41). On each subsequent laser pulse return 16, thepattern that is formed on the phosphor screen 15 is accumulated, so thatgenerally the digitized image of the phosphor screen 15 may representsignals accumulated from many laser pulse returns 16.

In an alternative embodiment, the lidar system 10 may emit a fanned beamof laser pulses 6 directly at the target 7 for a 3D representation ofthe target 7. However, in such an embodiment, the target may need to berelatively close. For example, for targets that are relatively far away,spatial determinations of the target may not be resolvable. So, a slitaperture may allow too much background light in the streak tube 11. Moregenerally, the aperture 17 may be dynamically configurable to permiteither mode of operation.

In one embodiment, the processor 26 may also provide calibration of thestreak tube system 10 by introducing fiducial signals. The fiducialsignals have known qualities or attributes that are operable to providea geometric calibration mapping of specific pixels to time and imageangle. For example, variations in environmental conditions (e.g.,changes in the orientation of the earth's magnetic field relative to thestreak tube 11, opto-mechanical distortion of the streak tube 11,thermal distortion of the streak tube 11, etc.) may be encounteredduring operation of the streak tube 11.

This “environmental degradation” can distort the geometric calibrationmapping of the electrons 18 to the phosphor screen 15 of the streak tube11. The embodiments herein can also provide a dynamic calibration thatcan directly/regularly measure and update geometric maps to account forthe environmental degradation during operations of the streak tube 11.The processor 26 may be operable to correct the environmentaldegradation of the streak tube 11 by adjusting the image generated byimaging optical element 21 and/or any other imaging components using thefiducial signals.

FIGS. 4 and 5 illustrates an exemplary lidar illumination of a sphericalobject (e.g., the target 7) via the optical pulses 6. The resultinglidar range image provides information on the general shape of theobject.

FIG. 4 shows an example of a received range image extracted from a laserpulse that has been reflected from the target 7. When interacting with atarget 7 that is extended in range, light that is scattered from thenear portions of the target 7 is received at an earlier time than lightthat is scattered from far portions of the target 7. The result is atemporal optical signal that is longer in duration than the transmittedpulse 6 and has a signal temporal shape that captures range dependentinformation of the target 7. In the case where the transmitted laserpulse 6 from the laser 8 has a range extent that is shorter than thetarget 7, the received signal from light scattered from the target 7 isoften referred to as a range image, as illustrated in FIG. 5. The rangeimage may be expressed as a function of time or, by multiplying the timeaxis by the speed of light and dividing by two, the range image may beexpressed as a function of range.

The lidar system 10 acts to spatially map the range image to thedetector 21. In one embodiment, the detector 21 comprises a plurality ofpixel detectors configured in an array that are associated with rangeimage time delays. In some embodiments, the detection mechanism may beselected to have a bandwidth that is much less than the laser repetitionrate. For example, phosphor emission times may be as long as many 10'sof ms. Consequently, the range image may be accumulated over many laserpulses. However, since the sweep pattern is periodic and synchronized tothe laser pulse rate, the accumulated signal at a pixel may beassociated with the same range relative to the target 7 on eachsubsequent laser pulse 6. This multiple pulse integration permits lowersingle laser pulse energies, since the extracted range image is theresult of integration over many laser pulse returns 16 (e.g., multiplescanning periods).

To resolve spatial information from the target 7, the pulse lengthshould have a duration that is shorter than the time it takes for theoptical pulse 6 to traverse the target 7. The ability to resolve spatialinformation from the target 7 improves with shorter pulses. While theembodiments herein are operable with any laser providing repeatableshort pulse lasers, there are particular advantages in using mode-lockedlasers. For example, lasers producing nanosecond class pulses typicallyrely on Q-switching techniques to generate optical pulses. Q-switchedlasers may be used in the lidar system embodiments herein because oftheir capability to generate high energy optical pulses with sufficientenergy for detection after propagation to and from a distant target 7.Optical pulse lengths for Q-switch lasers typically scale with thelength of the optical cavity and this typically results in nanosecondclass pulse lengths (or longer).

Shorter optical pulses can be achieved by using mode-locked lasers thatlock the phases of longitudinal laser cavity modes to produce opticalpulses that are much shorter than the optical cavity length. Mode-lockedlasers can be understood as generating a high intensity pulse thatcirculates in a laser cavity and releases an output pulse from theoutput cavity on each round-trip through the cavity. The time separatingeach pulse is the round-trip time of the cavity. The stability of thepulse frequency is generally very high. While the present embodimentsare operable with periodically pulsed Q-switched lasers, mode-lockedlasers are particularly well suited to some embodiments herein due totheir short pulse length and high stability.

Mode-locked lasers may be implemented as either solid state lasers withfree-space propagation or by using fiber propagation and fiber gainmedia. Either laser technology may be operable in one or more of theembodiments. However, for embodiments designed to interrogate targets,such as the target 7, that are more than a few meters in extent, thereare certain advantages for using mode-locked lasers that utilize fiberwithin their cavities.

FIG. 6 below illustrates an exemplary mode-locked fiber laser 50. Inthis embodiment, the laser cavity is a ring cavity 55 in which anoptical pulse circulates in a counter-clockwise direction 60. The lasermay, for example, include a saturable absorber (SA) 56, polarizationcontrol components 54, and a wave division multiplexer 53 to permitcoupling of pump radiation from a pump 52 into an active fiber of thering cavity 55. An isolator 51 may be used to suppress the buildup ofoptical pulses propagating in the clockwise direction. An output coupler57 emits a portion of the circulating optical pulse on each passing viathe optical element 59 (e.g., a lens and/or a dichroic mirror). Forexample, an optical pulse circulating through the ring cavity 55 maycause the emission of a first ultrashort laser pulse 6-1 followed by asecond ultrashort laser pulse 6-2 separated in time by a pulse width 62.

In some embodiments, the photo-cathode 14 in the streak tube 11 issensitive to radiation in the visible and ultraviolet (UV) opticalbands. If the mode-locked laser emits pulses in the infrared (e.g., ˜wavelengths near 1 μm, 1.5 μm, or 2 μm), the optical pulse may beconverted to visible or UV radiation through the use of a harmonicconversion component. Harmonic conversion methods and devices (e.g.,second harmonic conversion (SHC) or third harmonic conversion (THC)) canbe implemented with nonlinear optical crystals. In some embodiments, aYb-doped active fiber may be used within a mode-locked laser to producesub-ns optical pulses with wavelengths near 1 μm. In this regard, theharmonic generator 58 can be a SHC generator used to convert the 1 μminfrared pulses to green colored optical pulses. Alternatively, theharmonic generator 58 can be a THC generator used to convert 1 μm pulsesto UV emissions.

A fiber mode-locked laser has the advantage that the cavity length mayeasily be made much longer than the physical length of the laser system,for example, using coiled fiber. Consequently, the spatial separation ofsuccessively emitted laser pulses 6 can be made many meters in lengthwhile using a relatively compact laser. It may be advantageous to havethe spatial separation of successively emitted laser pulses exceed thespatial extent of an interrogated target 7 so that the receivedemissions 16 from subsequent laser pulses 6 do no not overlap in time.If subsequent laser pulses result in received signals that overlap intime, more complex processing may be needed to extract a range.

In some embodiments, a solid state laser may be used with a “pulsepicker” that permits emission of one pulse per some preselected numberof pulses, effectively emitting optical pulses at a subharmonic of theintrinsic cavity pulse rate. The pulse picker may be implemented withinthe optical cavity or outside of the optical cavity. In manyembodiments, a laser amplifier may additionally be included after themode-locked laser cavity to provide more energy to the generated opticalpulse stream. Typically, the amplifier amplifies optical pulses 6 beforeany harmonic conversion.

In the case where a target 7 has a velocity component (positive ornegative) with respect to the direction from the laser 8, a time delaymay be continuously added to the sweep voltage circuit 12, toeffectively provide Doppler correction to the laser repetition rate andpermit the previously described pulse integration without smearing fromtarget displacement on the phosphor screen 15.

Since the scan pattern generated on the phosphor is generallycurvilinear, and in some embodiments, a Lissajous pattern, imageprocessing methods may be used to extract a range image. A calibrationprocedure, in which a flat fiducial reflector produces a lidar returnand is successively repositioned in range, may be used to map rangeoffsets to specific pixels. For each time delay, weightings for one ormore pixels for each time step in the extracted range image may beassigned to sum up a range image signal strength. For example, pixelsthat are near the streak tube scan path, may be give weights forcalculating a signal associated with a specific position on the scanpath. This procedure is used to reduce aliasing effects in thecalculated measured range image.

For Lissajous sweep patterns, the sweep pattern may overlap itself, asillustrated in the Lissajous sweep pattern 70 of FIG. 7. At pixels 71that are near a crossing 73 in the sweep pattern 70, a pixel's signallevel may have contributions from an optical return signal level at twonon-contiguous segments of time, the “early path” 74 and the “late path”75. Consequently, there may be some ambiguity in assigning range imagevalues to those two non-contiguous time segments. In other words, pulsesreceived at different times may combine intensities on the phosphorscreen 15 at the crossing of sweep paths 74 and 75.

A two-dimensional hypothesis set may define the possible signals for thetwo crossing paths 74 and 75 at the intersection point 73, with theconstraint that a weighted sum of the signals (from the twonon-contiguous time segments) should be consistent with the measuredpixel intensities near the crossing. Each hypothetical pair of signalstrengths may be assigned a probability based on interpolations from thepixel signal values away from crossing 73 on the two sweep legs 74 and75. The pair of range image signals values having the highestlikelihood, based on the probability assignments for the twointersecting legs 74 and 75 may be selected when determining themeasured range image.

Alternatively or additionally, a time delay may be simultaneously addedto both the vertical and horizontal sweep waveforms (e.g., relative tothe laser pulse triggers). A time delay added to both waveforms does notchange the sweep pattern, but it may displace the range image data alonga curvilinear sweep path. The displaced range image data results in thespatial pattern crossing 73 corresponding to an ambiguity in the rangeimage at a different pair of non-contiguous time segments. The rangeimage at the non-contiguous time segments corresponding to the spatialcrossing 73 no longer correspond to the spatial crossing after a timedelay is added, removing the need for resolving the ambiguity. In otherwords, by occasionally dithering the time delay between the sweepwaveform(s) of the streak tube 11 and the laser pulse triggers (e.g.,from the laser 8 to the sweep voltage circuit 12), this “phase dithercrossing deconfliction” method may resolve the ambiguity.

In some embodiments, a time delay for the vertical and horizontal sweepwaveforms may be selected to shift the range image waveform within aLissajous sweep pattern 70 such that the signal level is low ornegligible at any pattern crossings. A time delay may also be chosensuch that the speed of the sweep provides a desired range resolutionduring portions of the range image having higher bandwidth spatialinformation.

In the case where the angle of target illumination undergoes rotation(e.g., due to movement of the interrogating laser 8 and/or angularmotion of the target 7), a sequence of high resolution range images maybe collected and accumulated over time. Using analysis and algorithmsdeveloped for computed tomography (CT) applications and for inversesynthetic aperture imaging, a series of range images may be used toconstruct a 2D image, or even a 3D image, of the target 7. Moregenerally, however, range image sequences may be used to generate target“fingerprints” for identification of targets without the need forgenerating actual images of the target 7.

Synthetic aperture imaging was first developed with radar systems (knownas “SAR”) to produce high-resolution radar reflectivity maps constructedfrom data accumulated from a moving platform. The approach takesadvantage of high-resolution range data observed from multiple vantagepoints to reconstruct a high-resolution image, even when transverseimaging resolution, limited by diffraction, is relatively low. Whilethere are many technical approaches for synthetic aperture imaging, manyrely on high-resolution range imaging and tomographical reconstruction.

Tomographical reconstructions work because a sensor is collecting anapproximation to a Radon transform (e.g., the mathematical model of a CTscan). FIG. 8 illustrates one such X-ray CT geometry. For example,imagine a single, relatively thin X-ray beam 82 passing through sometwo-dimensional distribution of matter 81, f(x,y) from an X-ray source85. Assume that the beam is oriented at angle θ (87) with respect to thex-axis and is distance ρ from the origin 83. The line L (86) thatdefines the X-ray beam 82 in the (x,y) plane has a distance ρ (84) fromthe origin 83. The line L (86) that defines the X-ray beam 82 in the(x,y) plane is L=x cos(θ)+y sin(θ)=ρ. The detector 21 will see theintegrated density distribution of matter along this line L (86). Inother words, the detector 21 sees

R(ρ, θ) = ∫_(L)f(x, y)ds = ∫_(−∞)^(∞)f(x, y)δ(x cos  θ + y sin  θ − ρ)dxdy,

where δ(t) is a Dirac delta function, R(ρ,θ) is called the RadonTransform of f(x,y).

Here, it can be shown that the one-dimensional Fourier Transform ofR(ρ,θ) with respect to ρ is equal to the two-dimensional FourierTransform of f(x,y). This is known as the Central Slice Theorem and ispart of CT-Scans as well as synthetic aperture imaging.

The Central Slice Theorem is demonstrated by taking Fourier Transform ofR(ρ,θ) with respect to ρ and recognizing the results as the Fouriertransform of f(x,y) evaluated at (ω cos θ; ω sin θ) as follows:

{circumflex over (R)}(ω;θ)=∫R(ρ,θ)e ^(−πiρω) dρ=∫∫f(x,y)e ^(−2πixcosθω)e ^(−2πiysinθω)dxdy

As ω varies, a line with angle θ (87) to the u-axis in the (u,v) planeis tracked out. And, a slice of the Fourier transform through the centerof the (u,v) plane is produced.

While a CT scanner measures a Radon transform with transmitted “pencilbeams”, the geometry for SAR or synthetic aperture ladar (SAL) systemsis different since the interrogating radiation is larger than themeasured features. For SAR and SAL, the measured spatial resolution ofthe measurement in ρ is from time-of-flight backscattered lightpropagating along the ρ{circumflex over ( )}-direction instead ofrelative to the laser 8 and source translation in the ρ direction.

FIG. 9 illustrates the direction of the propagating radiation and anaccumulated Radon transform for a specific angle associated with thedirection of the signal. The one-dimensional Fourier transform of therange waveforms accumulated by a Radar or lidar system assembled overall interrogation angles of a scene 90 is approximately equal to the 2DFourier transform of the scene. In practice, synthetic aperture imagingsystems (e.g., SAR and SAL) make measurements over limited angularextents, and the obscurations from multiple features within the scene 90limit Radon function measurements.

FIG. 10 demonstrates the effect of measurements over a limited angularextent. The scene 90 is depicted at the top of FIG. 10. The Radontransform 95 is collected at one degree increments. Two images 91 and 93are reconstructed (e.g., using filtered back-projection). The rightimage 91 has all of the angles and the left image 92 has only half. Theboxes 93 in the lower left-hand spectrum show the spatial frequenciesthat are used in the partial (e.g., 90°) reconstruction. The non-zerospatial frequencies outside of the boxes 93 occur because of frequencies“spraying” into those areas due to interpolation done by the inverseprocessing.

Using the central slice theorem and given the measured lidar range imageover aspect, R(ρ,θ), a method to reconstruct the image, f(x,y) ispresented. Conceptually, one would populate the (u,v) plane by computing{circumflex over (R)}(ω;θ) for all θs and then take an inverse 2DFourier transform. However, this approach has interpolation issues andit is not how the Radon transform is typically inverted in practice. Thetechnique most commonly discussed in introductory approaches is called“filtered back-projection”.

Regardless of the reconstruction algorithm, the resolution of the targetimage is dependent on the volume of data in the spatial spectrum domain.For the 2D examples, the volume of spectral content is proportional tothe angular extent of the measurement, but with a quadratic dependenceon the spectral content in the range measurement. It follows that, byhaving finer range resolution, objects can be tomographicallyreconstructed with less angular diversity in the measurements.

Tomographical reconstruction of the target 7 may be performed to produceeither a 2D representative cross-section or with greater angle diversityabout two angular axes. While in some embodiments, the representationmay be suitable for human recognition of the target 7, in otherembodiments, data signatures of the target 7 may act as a suitablefingerprint for automated target identification without requiring a fullcalculation of a 2D or 3D structure. Moreover, the collected datacontains spatial information content about physical structures of thetarget 7 that can be analyzed and compared to known target structuresand to permit target identification and to detect target changes.

For example, a particular object may have certain signatures. Thus, whenthe processor 26 determines that certain metrics of the object have beenmet, the processor 26 may identify the object without having tocompletely construct an image of the object itself. This may reduce theamount of processing required to correctly identify the target such thatprocessing capabilities can be used elsewhere (e.g., for identifyingother targets in the scene 90).

Coherent sensing approaches can provide wavelength range resolutionspermitting target reconstructions with measurements over small angularchanges, but at a price as they have challenging sensing and processingrequirements. Conventional direct detection methods have much simplerimplementations and processing requirements more suitable for dynamictarget scenarios, but require substantial variations in target lookangles to construct a target image. The present embodiments permit muchfiner range resolutions with direct detection and more efficient/simplerlaser architectures which lead to target reconstruction with lessangular extent in measurements from a platform with lower Size, Weight,and Power (SWaP).

FIG. 11 is a block diagram of another exemplary lidar system 100operable to implement one exemplary process pertaining to lidarinterrogation (e.g., as illustrated in FIG. 12). The lidar system 100fires laser pulses 6 at the target 7. The lidar system 100, in thisembodiment comprises a laser cavity 104. Optical pulses are generatedwithin the cavity 104, which circulates the pulses until they areemitted by an output coupler 106. The cavity 104 may include anycombination of mirrors, gain mediums, polarization components, andlenses, as desired, to generate a pulse and emit it from the cavity.

For example, the output coupler mirror 106 may be a dichroic mirror thatallows the pump laser 101 to pump the laser cavity 104 at one wavelengthand allow the laser pulse 6 to exit the laser cavity 104 at a differentwavelength. In this regard, one or more of the optical elements 102,103, and 105 may be operable to change the polarization of the laserpulse 6 such that it may pass through the output coupler mirror 106.Alternatively, a wave plate may also be configured within the lasercavity 104 that alters the polarization of the laser pulses 6.

Laser pulse returns 16 are received at a streak tube imaging system 107of the lidar system 100. The streak tube imaging system 107 transformsthe returns 16 into 2D images which are subsequently processed by theprocessor 26 to calculate range images. The processor 26 generatesspatial structure information 110, such as target radon transforms, 2Dimages, 3D images, and/or other identifying traits of the target 7.

In one embodiment, the pump laser 101 is operable to pump the laserlight into the laser cavity 104. The laser cavity 104, in thisembodiment, provides mode locking for the laser energy so as to generateultrashort laser pulses 6 at high PRFs. As used herein, an ultrashortlaser pulse generally refers to any laser pulse duration that is lessthan one nanosecond. The laser cavity 104 includes, for example, amode-lock element (e.g., optical element 103, a saturable absorber), again medium 105, and an output coupler mirror 106. Certain exemplaryembodiments provide for pulse durations in the nanosecond range (orshorter, such as picosecond range) with PRFs of about 50 MHz or higher.For example, the PRF may be configured at a rate that permits many laserpulses to be in the air, between the lidar system and the target, all atthe same time.

Generally, the reciprocal of the PRF (i.e., the pulse separation period)is configured to be at least greater than two times the range extent ofthe target 7 divided by the speed of light

$\left( {{e.g.},{\frac{1}{PRF} > \frac{2\Delta \; r_{targ}}{c}},} \right.$

where Δr_(targ) is the length of the illuminated target and c is thespeed of light). In some cases, a higher PRF can be selected, but thismay result in the range image wrapping around the sweep cycle (e.g.,scanning period) of the streak tube 11, and additional algorithms may beneeded to extract the target information, as described above in FIG. 7.

The streak tube imaging system 107 comprises a streak tube, such asstreak tube 11 of FIG. 1, that measures light intensity variations oflaser pulses over time. That is, the streak tube imaging system 107measures the temporal evolution of optical power, or intensity, of laserpulse returns 16. In this embodiment, the streak tube imaging system 107is operable to perform that functionality on the laser pulse returns 16from the target 7. In this regard, the streak tube imaging system 107may be “triggered” with the firing of the laser pulses 6 so as to ensurethat the periodic laser pulse returns 16 are properly recorded. Thestreak tube imaging system 107 operates by transforming the intensityvariations of a laser pulse return 16 into a spatial profile on adetector (e.g., a charge coupled device camera, or “CCD camera”) andcausing a time-varying deflection of the light within the detector,called the “streak image”.

Streak tubes in general employ an elongated horizontal slit. However, insome embodiments, the aperture may not be elongated (e.g., havingsimilar horizontal and vertical extent) where the laser pulse returns 16enter, as illustrated in FIG. 1. This may provide the lidar system 100with improved range image resolution during horizontal sweeps while alsoreducing background optical collection.

Photons reaching a photo-cathode of the streak tube imaging system 107are converted to electrons via the photoelectric effect. These electronsare accelerated longitudinally within the streak tube imaging system 107by a bias grid. At the end of this longitudinal acceleration, theelectrons are focused by an electrostatic lens. Electrons are alsodeflected laterally by a time-varying transverse electric field. Thisfield is created by placing electrical potentials across one or twopairs of plates (each pair having one plate on each side of the electronstream). This effectively paints the electron stream onto a phosphorscreen of the streak tube imaging system 107 in a sweeping motion(similar to a cathode-ray tube television). The phosphorescence from thephosphor screen (i.e., the streak image) is then recorded by thedetector.

The streak tube imaging system 107 may be mechanical, optoelectronic, ora combination thereof. Mechanical streak cameras are somewhat limiteddue to the speeds at which their rotating mirrors operate. Thus,optoelectronic streak tube imaging systems may be used for faster PRFsystems with shorter pulse durations. Optoelectronic streak tube imagingsystems operate by directing laser pulse light onto a photo-cathode,which when hit by photons produces electrons via the photoelectriceffect. The electrons are accelerated in a cathode ray tube (a.k.a. astreak tube) and pass through an electric field produced by one or twopairs of plates. For example, a first set of plates may deflect theelectrons in a perpendicular manner and in some systems a second set ofplates may deflect electrons in a horizontal direction. By modulatingthe electric potential between the plates, the electric field is quicklychanged to give a time-varying deflection of the electrons and sweepingthe electrons across the phosphor screen at the end of the cathode raytube. In some embodiments, an optoelectronic streak tube imaging systemcan achieve temporal resolutions on the order of 100 femtoseconds orless.

The implementation of the streak tube imaging system 107 in the lidarsystem 100 may be made as a matter of design choice. However, asrelatively high resolution of the imaging of the target 7 may benecessary (e.g., in relatively long range target environments), certainembodiments of the lidar system 100 may favor the use of ultrashortlaser pulses with relatively high PRFs.

The processor 26 is any device, system, software, or combination thereofoperable to generate a representation (e.g., a 2D image, a 3D image,and/or other forms of identification) of the target 7 from the 2D imagesof the laser pulse returns 16 produced by the streak tube imaging system107. One example of a computing system operable as the processor 26 isshown and described below in FIG. 21.

FIG. 12 is a flowchart illustrating an exemplary process 150 of thelidar system 100 of FIG. 11. In this embodiment, the lidar system 100fires ultrashort laser pulses from a laser (e.g., a mode-locked laser),in the process element 151. As mentioned, the lidar system 100 may beconfigured as a high resolution lidar system. Part of the reason forthis regards the range to the target 7 and the angular diversity of thetarget 7 with respect to the lidar system 100. For example, the lidarsystem 100 may be capable of imaging objects over great distances (e.g.,over a kilometer, 100 km, or even 1 Mm). And, a large number ofpulses/returns may be necessary to produce such images.

Additionally, a large number of pulses at a relatively high PRF (e.g.,80 MHz or higher) allows the lidar system 100 to sample differentportions of the target 7 due to the angular diversity between the target7 and the lidar system 100. As illustrated in the process element 151,the lidar system 100 fires the ultrashort laser pulses 6 at the target 7during a plurality of scanning periods. For example, the streak tubeimaging system 107 is operable to provide a 2D image of intensityvariations of a laser pulse return 16 over time by transforming theintensity variations of the laser pulse return 16 into a spatialprofile. Instead of observing the intensity variation of a single laserpulse return 16 over time, the phosphor screen of the streak tubeimaging system 107 can be used to accumulate intensity variations ofmultiple laser pulse returns 16 over time, essentially integrating theintensity variations of a plurality of laser pulse returns 16. The lidarsystem 100 takes advantage of this phosphorescence of the streak tubeimaging system 107 to generate a scanning period from each of a multiplelaser pulse returns 16.

In this regard, the streak tube imaging system 107 collects returns 16of the ultrashort laser pulses 6 from the target 7 during each scanningperiod, in the process element 152. And, during each scanning period,the streak tube imaging system 107 generates a 2D image of theultrashort laser pulse returns 16 from the target 7, in the processelement 153. The processor 26 then processes the 2D images generated bythe streak tube imaging system 107 to generate spatial structureinformation 110 of the target 7, in the process element 154. Forexample, the processor 26 may generate a representation of the target 7,such as a 2D image, a 3D image, or some other signature of the target 7,that may be used to identify the target 7.

It should be noted that in the embodiments shown and described herein,the target 7 may be moving with respect to the lidar system 100.However, the lidar system 100 may be alternatively or additionallymoving with respect to the target 7 so as to produce the desired angulardiversity between the lidar system 100 and the target 7.

FIG. 13 is an exemplary block diagram of a mode-locked laser cavity 104.Again, the laser cavity 104 includes, for example, a mode-lock element103 (e.g., a saturable absorber), a gain medium 105, and an outputcoupler mirror 106. Generally, the laser cavity 104 is operable tooutput the laser pulses 6. The laser cavity 104 has a length L that maybe determined as a matter of design choice. For example, a laser pulse 6exits the laser cavity 104 at the output coupler mirror 106. The laserpulse 6 is emitted from the laser cavity 104 when a laser pulse withinthe cavity is incident on the output coupler mirror 106. Thus, the timebetween laser pulses is the time it takes for internal pulses tocirculate the laser cavity 104. For linear cavities this time is 2 L/c,where c is the speed of light (˜3×10⁸ m/s). So, if one desired to have aPRF of 80 MHz, the laser cavity 104 may be configured to be about 1.875meters in optical path length for a linear cavity, or 3.75 m for a ringcavity.

FIG. 14 illustrates an exemplary series of pulses 6-1-6-3 from themode-locked laser cavity 104 of FIG. 13. The pulses 6-1-6-3 have a pulseseparation of 2 L/c (note that for a ring cavity the pulse separationmay be L/c). In embodiments that utilize a mode-locked fiber laser, thecavity length may be much longer than the physical length of the lasercavity and this can allow the lidar system to unambiguously imagetargets that are much larger than the size of the laser itself.Alternatively, by selective pulse picking, the time period betweenpulses can be chosen to enable detection of longer targets withoutincurring range image wrapping around the sampling period.

Again, the embodiments herein are not intended to be limited to anyparticular laser cavity length or PRF as such may be configured as amatter of design choice. In fact, a cavity end mirror (e.g., the opticalelement 102) may also be configured with an actuator that changes thelength of the laser cavity 104 as desired. For example, detection oflaser pulses in the picosecond or even femtosecond range at relativelyhigh PRFs is a challenging proposition. The streak tube imaging system107 advantageously provides the ability to image temporal profiles ofultrashort laser pulses at high PRFs. But, the PRF may also be modulatedby the actuator over a scanning period so as to compensate for Dopplershifts in the received signal. For a fast rotating target, Dopplershifts of signals emanating from different portions of the target mayresult in a range image “smearing-defocus” in an integrated image. Amodulated PRF may be used to provide sequential compensation todifferent portions of the range image.

The frequency difference between the PRF of the voltage waveformsprovided to the streak tube imaging system 107 and the PRF of lasercavity may be iteratively modified to improve Doppler compensation. Ameasurement metric indicative of relatively good Doppler correction canbe used to determine whether the difference frequency should beincreased or reduced. An exemplary metric is the range image signalstrength within a specified frequency band. When Doppler differencefrequencies are incorrect, the collected range images may include a“smear defocus” that reduces the high frequency spectral content.Incremental frequency corrections to increase the higher spectralfrequency components of the measured range image may be used to improveDoppler corrections.

FIG. 15 exemplarily illustrates laser pulses 6-1-6-3 and theircorresponding returns 16-1-16-2 (with laser pulse return 16-3 not beingshown for the sake of simplicity) from an approaching target 7. Thelaser pulse returns 16 may have a temporal phase mismatch with respectto their corresponding laser pulses 6. As can be seen from the graph(exaggerated for the purposes of illustration), the laser pulse return16-1 has a larger delay x from its corresponding laser pulse 6-1 thandoes the laser pulse return 16-2 from its corresponding laser pulse 6-2(i.e., delay y). This indicates that the sampling time between thereceived return signals 16-1 and 16-2 is less than the time between thetransmitted laser pulses 6-1 and 6-2. Thus, it can be concluded that themoving target has changed position. However, since the PRF of the lidarsystem 100 is relatively high, in some scenarios, it may be determinedthat the actual change of position is somewhat negligible and that thechange in distance may also be attributable to rotation or motion withinthe target 7. During multiple cycle time integration on the phosphorchanges in the delay result in a “smear defocus” of the range image. Thesmear defocus can be reduced by implementing a corrective Dopplerfrequency mismatch between the laser PRF and the streak tube RF waveformfrequency used to drive the electron sweeps.

FIG. 16 illustrates a series of received laser pulse returns 16 and theresulting increase in phosphor brightness in the streak tube imagingsystem 107. On a time scale, s, that is shorter than the phosphoremission time scale (τ_(phosphor)), the phosphor brightness is increasedwith each subsequent optical signal iteration (e.g., from laser pulsereturns 16-1-16-4), effectively integrating the return signal (e.g., ateach specific phosphor spatial location). At time-scales longer than thephosphor emission time, equilibrium is achieved and the phosphorbrightness is proportional to the average power absorbed from incidentelectrons. The phosphor acts to provide analog signal integration foreach time element in a repetitively received range image. Theequilibrium phosphor brightness may then be further integrated by acamera (e.g., the detector 21) which collects an image over a time thatmay be selected as comparable to the phosphor emission time scale.

FIG. 17 is a block diagram of another exemplary lidar system 100employing a supplemental detection system 130. The detection system 130may be used to initially detect the target 7 so as to provide guidanceto the lidar system 100. For example, the lidar system 100 may beoperable to image targets from greater distances. However, thosedistances may not be readily observable to the human eye. And, the lidarsystem 100 is highly directional. So, knowing where to point the lidarsystem 100 can be a challenge. The detection system 130 may beimplemented in a variety of ways as a matter of design choice toovercome the challenge. For example, one possible detection system maybe a radar system that is operable to initially detect the target 7 asit enters within range of the lidar system 100. With its locationdetermined, the lidar system 100 could then image the target 7 asdescribed in the embodiments above. The supplementary detection systemmay, for example, provide a velocity of approach that may be used toimplement a laser and streak camera waveform PRF mismatch for Dopplercompensation.

In yet another embodiment, a lidar system 160 shown in FIG. 18 includesa laser 161 that may generate optical pulses at an infrared wavelength(e.g., 1030 nm) and utilize harmonic generation 162 to convert much ofthe energy in the infrared pulses into UV or visible optical pulses(e.g., 515 nm, via second harmonic generation) so that pulses at boththe fundamental wavelength and the harmonic wavelength are delivered toa target, such as target 7 of FIG. 1. The detection system 163 mayutilize an infrared detector for measuring infrared returns from thetarget and determines the Doppler shifted return frequency therefrom.Determining the Doppler shifted return frequency may include the use ofa phase lock loop detection system. An electronically generated signalat the Doppler shifted return frequency may then be matched to thereceived infrared signal and used to control the streak tube deflectorplate voltage signals.

In some embodiments, the streak tube may be sensitive enough to detectsingle photons. In this regard, a single image of the phosphor may havea low number of bright spots corresponding to single photon detections.To limit electrical noise, each spot within the image that exceeds adetermined threshold may be identified as a single photon detection, andthe position of the spot relative to the sweep path may be used to add aphoton count to an accumulated digital range image. By repeating theprocess with multiple streak tube images, the accumulated digital rangeimage may have sufficient signal for further processing or analysis.

FIG. 19 shows a series of relatively small plastic dinosaurs 170 thatwere rotated as targets in one exemplary embodiment of the lidar system.In this exemplary experimental embodiment, the lidar system included aTi: Sappphire mode-locked laser that emitted ultrashort pulses at afrequency of approximately 79 MHz. The optical pulses were passedthrough a second harmonic generation crystal to produce optical pulseswith a wavelength of 400 nm which were then projected to illuminate adinosaur target 170 while being rotated above a turntable at rotationperiod of about 49 seconds. A streak tube was operated with vertical andhorizontal electrode voltages that were matched to the laser pulsefrequency with a phase locked loop. Using 100 ms integration times onthe camera, the phosphor was imaged once for approximately every 8million optical pulses. In this mode of operation, the streak tubetypically captured between several 10's of photons and a few hundredphotons for each camera image. Using thresholding and blob detection,each photon detection was identified and replaced in the image planewith a single count in a pixel. This image processing based photondetection process vastly reduces system noise.

As shown in FIG. 20, range images were extracted from each photoncounting processed image frame and stacked to form a sparse target radontransform 181. Prior to calculating an inverse radon transform 183 toextract the 2D target images, a low pass spatial filter 182 was used toprovide some effective signal integration over small angles and smallranges. For the inverse radon transform 183 to produce a focused 2Dimage, the range about which the target was rotated should be known.Series trial rotation ranges were hypothesized and tuned to provide aninverse radon transform (calculated through filtered back-projectionalgorithms) that had “in-focus” properties suitable for interpretationas an image. Next to the image of each plastic dinosaur 170 in FIG. 19,the resulting tomographically reconstructed target image 171 is alsoshown. This exemplary LIDAR embodiment illustrates the high resolutiontarget reconstruction capability of the embodiment, even when low lightreturns are collected and for methods that do not require spatiallyresolved imaging. The exemplary embodiments herein are very suitable forhigh resolution imaging of targets at very long ranges.

Any of the above embodiments herein may be rearranged and/or combined asa matter of design choice. Accordingly, the embodiments are not to belimited to any particular embodiment disclosed herein. Additionally, theembodiments can also take the form of an entirely hardware embodiment oran embodiment containing both hardware and software elements. In oneembodiment, portions of the embodiments are implemented in software(e.g., to be processed at least in part by the processor 26), whichincludes but is not limited to firmware, resident software, microcode,etc. FIG. 21 illustrates a computing system 200 in which a computerreadable medium 206 may provide instructions for performing any of themethods disclosed herein.

For the purposes of this description, the computer readable medium 206can be any apparatus that can tangibly store the program for use by orin connection with the instruction execution system, apparatus, ordevice, including the computer system 200. The medium 206 can be anytangible electronic, magnetic, optical, electromagnetic, infrared, orsemiconductor system (or apparatus or device). Examples of a computerreadable medium 206 include a semiconductor or solid state memory,magnetic tape, a removable computer diskette, a random access memory(RAM), a read-only memory (ROM), a rigid magnetic disk, a solid statestorage device (SSD), and an optical disk. Some examples of opticaldisks include compact disk-read only memory (CD-ROM), compactdisk-read/write (CD-R/W) and digital versatile disks (DVD).

The computing system 200, suitable for storing and/or executing programcode, can include one or more processors 202 coupled directly orindirectly to memory 208 through a system bus 210. The memory 208 caninclude local memory employed during actual execution of the programcode, bulk storage, and cache memories which provide temporary storageof at least some program code in order to reduce the number of timescode is retrieved from bulk storage during execution. Input/output (I/O)devices 204 (including but not limited to keyboards, displays, pointingdevices, etc.) can be coupled to the system either directly or throughintervening I/O controllers. Network adapters may also be coupled to thesystem to enable the computing system 200 to become coupled to otherdata processing systems, such as through host systems interfaces 212, orremote printers or storage devices through intervening private or publicnetworks. Modems, cable modem and Ethernet cards are just a few of thecurrently available types of network adapters.

What is claimed is:
 1. A laser ranging and detection (lidar) system,comprising: a laser operable to propagate ultrashort laser pulses to atarget during a plurality of scanning periods; a streak tube imagingsystem operable to collect returns of the ultrashort laser pulses fromthe target during each scanning period, and to generate atwo-dimensional (2D) image of the returns of the ultrashort laser pulsesduring each scanning period; and a processor operable to generate arepresentation of the target based on the 2D images from the streak tubeimaging system.
 2. The lidar system of claim 1, wherein: the processoris further operable to synchronize the propagation of at least a portionof the ultrashort laser pulses from the laser with a correspondingportion of the returns of the ultrashort laser pulses at the streak tubeimaging system.
 3. The lidar system of claim 1, wherein: the laser is amode-locked laser operable to propagate the ultrashort laser pulses at arate of at least 1 MHz.
 4. The lidar system of claim 1, furthercomprising: a radar system operable to detect a location of the target,and to direct the laser to the target.
 5. The lidar system of claim 1,wherein: the processor is further operable to correct a Dopplerfrequency mismatch between laser pulse repetition frequency and afrequency of a voltage waveform of the streak tube imaging system. 6.The lidar system of claim 1, wherein: the laser comprises a cavity withat least one gain medium.
 7. The lidar system of claim 1, wherein: thestreak tube imaging system is operable to integrate the returns of theultrashort laser pulses over multiple scanning periods.
 8. The lidarsystem of claim 1, wherein: the processor is further operable tooperable to perform a tomographical reconstruction of the target.
 9. Thelidar system of claim 1, wherein: the streak tube imaging systemcomprises two electrode pairs, each electrode pair coupled to adedicated time varying voltage waveform source, to sweep electronsresulting from the returns of the ultrashort laser pulses in acontinuous pattern.
 10. The lidar system of claim 1, wherein: theprocessor is further operable to resolve ambiguities in crossing sweeppatterns of the streak tube imaging system.
 11. The lidar system ofclaim 1, wherein: the processor is further operable to perform aninverse Radon transform to generate the representation of the targetfrom the 2D images from the streak tube imaging system.
 12. A laserranging and detection (lidar) method, comprising: propagating ultrashortlaser pulses from a laser to a target during a plurality of scanningperiods; collecting returns of the laser pulses from the target duringeach scanning period with a streak tube imaging system; generating atwo-dimensional (2D) image of the returns of the ultrashort laser pulsesduring each scanning period; and generating a representation of thetarget based on the 2D images.
 13. The method of claim 12, furthercomprising: synchronizing the propagation of at least a portion of theultrashort laser pulses from the laser with a corresponding portion ofthe returns of the ultrashort laser pulses at the streak tube imagingsystem.
 14. The method of claim 12, wherein: the laser is a mode-lockedlaser; and the method further comprises propagating the ultrashort laserpulses at a rate of at least 1 MHz.
 15. The method of claim 12, furthercomprising: detecting a location of the target with a radar system; anddirecting the laser to the target based on said detecting.
 16. Themethod of claim 12, further comprising: correcting a Doppler frequencymismatch between laser pulse repetition frequency and a frequency of avoltage waveform of the streak tube imaging system.
 17. The method ofclaim 12, wherein: the laser comprises a cavity with at least one gainmedium.
 18. The method of claim 12, further comprising: integrating thereturns of the ultrashort laser pulses from the target over multiplescanning periods with the streak tube imaging system.
 19. The method ofclaim 12, further comprising: performing a tomographical reconstructionof the target.
 20. The method of claim 12, further comprising: sweepingelectrons resulting from the returns of the ultrashort laser pulses in acontinuous pattern via two electrode pairs of the streak tube imagingsystem, wherein each electrode pair is coupled to a dedicated timevarying voltage waveform source.
 21. The method of claim 12, furthercomprising: resolving ambiguities in crossing sweep patterns of thestreak tube imaging system.
 22. The method of claim 12, furthercomprising: performing an inverse Radon transform to generate therepresentation of the target from the 2D images from the streak tubeimaging system.
 23. A laser ranging and detection (lidar) system,comprising: a laser operable to propagate laser pulses to a target; astreak tube imaging system operable to collect returns of the laserpulses from the target, to propagate electrons resulting from thereturns to a phosphor screen using a pair of time varying waveforms, andto generate two-dimensional (2D) images of the returns based on theelectrons; and a processor operable to generate a representation of thetarget based on the 2D images.